1. Field of the Invention
The present invention relates to an underground borer with down hammer and more particularly to an underground borer in which a fluid jetting pipe is arranged centrally of a down hammer of the borer so that fluid may be discharged, along with compressed air, from the bottom portion of a bit body to the outside of the borer.
2. Description of the Prior Art
Known for a conventional underground borer with down hammer or a method of construct ion using the same is a technique disclosed in Japanese Patent Application Laid-open No. Sho 55-119885 wherein a hammer excavating head is coupled to a ram of an oil pressure cylinder suspended so as to be free to ascend and descend, a circuit for feeding pressurized oil is connected to the oil pressure cylinder and that circuit is connected with a high pressure relief valve and with a low pressure relief valve through a switching valve, whereby by determining setting pressures of the two relief valves to proper values, ground pressure received by a spiking bit can be adjusted to a value which allows the spiking bit to move vertically; or a technique disclosed in Japanese Utility Model Application Laid-open No. Sho 55-113786 wherein an air hammer having a spiking bit attached to the fore end of a screw auger is mounted on its outer periphery with a spiral screw blade whose diameter is increased upwardly gradually and the screw blade is mounted with an excavating cutting edge, whereby even with a small-diametered or size-reduced air hammer, a hole of a large diameter can be excavated. A method of construction for boring has also been known as disclosed in Japanese Patent Application Laid-open No. Sho 55-23272, according to which in a borer wherein a piston operated by compressed air is attached to the bottom of a drill pipe and a bit is mounted to the piston so as to be moved vertically, a nozzle having a check valve directed laterally is disposed in the lower end of the drill pipe so that as excavation effected by the borer proceeds, mortar or the like is blown from the nozzle against a bored hole wall so as to be forcibly deposited thereon.
In the conventionally known underground borers the bit body inside the down hammer reciprocated under the application of compressed air to bore a rock by the hammer act ion but heat is generated by friction between the bit body and the rock to raise the temperature of the bit body and as a result anti-wear properties of the bit body is degraded within relatively short period of time. In addition, since powdery dirt is are generated in the course of boring to pollute workers as well as environmental circumstances and the hammer in operation generates a roaring noise, reduction of powdery dirt and reduction of noises are needed. Further, the boring speed is not always satisfactorily high.